1. Field of the Invention
The present invention relates to a method for wireless data transmission between a base station and a transponder, for example, a backscatter-based transponder, in which the transponder generates a presence signal after initialization or at prescribable time intervals, independent of data transmitted by the base station.
2. Description of the Background Art
Transponders are used in radio frequency identification (RFID) systems. In this case, data are transmitted bidirectionally in a wireless manner between one or more base stations or readers and one or more transponders. Sensors, for example, for temperature measurement, can also be integrated into the transponder. Such transponders are also called remote sensors.
Transponders or their transmitting and receiving devices typically do not have an active transmitter for data transmission to the base station. Such inactive systems are called passive systems, when they do not have their own power supply, and semipassive systems, when they have their own power supply. Passive transponders draw the operating energy or operating power necessary for their supply from the electromagnetic field emitted by the base station.
For data transmission from a transponder to a base station with UHF or microwaves in the far field of the base station, as a rule, so-called backscatter coupling is employed. To that end, the base station emits electromagnetic carrier waves or a carrier signal, which is modulated and reflected by the transmitting and receiving device of the transponder by a subcarrier modulation process in accordance with the data to be transmitted to the base station. Amplitude modulation and phase modulation are the typical modulation processes for this purpose.
Data transmission is normally based on the so-called reader-talks-first principle, in which data transmission, also that from the transponder to the base station, is initiated by the base station or the reader. To that end, the base station typically sends data or a command to the transponder, which transmits its data subsequent thereto or overlapping to the base station.
Recently, transponders have also been used in electronic product security systems or electronic article surveillance (EAS) systems. Transponder-based EAS systems for this purpose typically comprise transponders, which are connected to the articles to be secured, and one or more base stations. If the article and thereby the transponder are brought into the transmission range or response range of a base station, it is detected by the base station and accordingly evaluated, for example, by the generation of an alarm signal.
To enable the most delay-free detection possible of a transponder by a base station, EAS functions of said type are based on the so-called tag-talks-first principle. The transponder or the tag hereby after initialization or at prescribable time intervals, independent of the base station, generates a presence signal, which is detectable by the base station, as soon as the transponder enters the transmission range or response range of the base station.
If transponders, which are used in EAS systems and generate a presence signal according to the tag-talks-first principle, are to be accessed according to the reader-talks-first principle, for example, to program them, it is necessary that the transponder can receive data from the base station, while it transmits the presence signal.
Data transmission from the base station to a transponder typically occurs with the use of amplitude modulation of the carrier signal transmitted by the base station in the form of successive field gaps or field attenuations of the carrier signal, which are also called notches. Notches of said type are detected in the transponder by using receiver signal strength indicator (RSSI) circuits.
If the presence signal of the transponder is generated by amplitude modulation and backscattering of the carrier signal by the transponder, the real part of an input impedance of the transponder is changed. In this regard, the modulation index is typically relatively large to achieve accordingly large signal-noise ratios of the backscattered signal. The change in the real part of the input impedance leads to a match or mismatch of the input impedance of the transponder in regard to an impedance of a transponder antenna, as a result of which the reflection or receiving properties of the transponder antenna change and thereby more or less power is reflected or received by the transponder or its antenna. These fluctuations, caused by the transponder itself, in the amplitude of the received carrier signal, however, can be differentiated only with considerable effort from the notch signals generated by the base station, which also result in a change in the amplitude of the received carrier signal. This applies in particular when the field attenuation for notch generation is to be kept low in order to increase the transmission range. Expensive input filters in the transponder are then necessary for the differentiation. To achieve differentiation by a filter, in addition the sidebands of the received and backscattered signal must be relatively far apart. Often, this cannot be realized because otherwise the relevant standards would be violated.